1. Field of the Invention
This invention relates to a wave gear drive, and more particularly to the shape of a wave generator used in a wave gear drive.
2. Description of Related Art
A wave gear drive typically consists of a rigid circular internal gear, a flexible external gear which has 2n (n being a positive integer) less teeth than the internal gear and which is disposed inside the internal gear and flexed into an elliptical shape so as to mesh with the internal gear at two places, and a wave generator fitted inside the external gear for flexing it. Rotating the input shaft of the wave generator rotates the elliptical shape of the external gear and causes the external gear and the internal gear to rotate relative to each other in proportion to the difference in the number of their teeth. If one of the gears is fixed and the other is connected with an output shaft, the output shaft will rotate at a greatly reduced speed relative to the input shaft. Because of this ability to produce a large reduction ratio with only a small number of machine elements, wave gear drives are widely applied in precision machines and the like.
Although the basic tooth profile for the gears of a wave gear drive is linear (see U.S. Pat. No. 2,906,143), wave gear drives using other profiles have also been developed. For example, Japanese Patent Publication Sho 45-41171 discloses one using involute gears. In addition, for increasing load capacity the present inventor in Japanese Patent Public Disclosure Sho 63-115943 proposed a system using as the basis for the tooth face profile the curve obtained by similarity transforming the locus of motion, at a reduction ratio of 1/2, over a prescribed range from the meshing limit point on the locus of motion based on the rack approximation of the tooth of the external gear relative to the internal gear determined by the shape of the wave generator. Moreover, in Japanese Patent Public Disclosure Sho 64-79448 the inventor proposed a system for expanding the meshing range by applying the foregoing system to a negative-deviation wave gear drive. The two wave gear drives proposed by the inventor referred to above both employ a system in which the gear tooth profiles are continuously meshed.
The wave gear drives according to U.S. Pat. No. 2,906,143 and Japanese Patent Publication Sho 45-41171 have a drawback in that the gears cannot be continuously meshed. On the other hand, while the gears of the wave gear drives according to Japanese Patent Public Disclosure Sho 63-115943 and Japanese Patent Public Disclosure Sho 64-79448 mesh continuously, no consideration is given either to the addendum interference arising at the beginning of meshing because of deformation under the load on the tooth face or to the amplitude of the tooth rim bending stress, which directly affects the strength of the flexible external gear. Thus none of the prior art wave gear drives actually achieves ideal meshing and all leave room for improvement in terms of the drive's load capacity.
The object of this invention is therefore to provide a wave gear drive which, owing to the improved shape of its wave generator, exhibits increased load capacity and achieves better meshing of the tooth profiles.